Use libwebp pc module instead of libs

[dg0yt]

WebP now provides a pkg-config module.

[vapier]

what version of libwebp though? we've been supporting older versions that distros are shipping.

[dg0yt]

what version of libwebp though? we've been supporting older versions that distros are shipping.

It works with 1.2.4 in vcpkg. IIRC webp in Ubuntu 18.04 is too old. webp added pkg-config support 4 years ago with 1.0.1:
webmproject/libwebp@eee00b6

If you want to retain compatibility with old versions of webp, a pkg_check_modules might be added to FindWEBP.cmake. (And vcpkg uses pkg-config even with MSVC.)

[vapier]

our Autotools build already checks pkg-config and supports webp 0.2.0+. so cmake should be comparable.

[dg0yt]

IIRC webp in Ubuntu 18.04 is too old.

I was wrong, even Ubunto 18.04 has it, as well as older Debian packages (0.6.1).

[vapier]

i'll note that Ubuntu 16 & 14 LTS are still supported & actively used

the autotools generated .pc DTRT when it comes to finding libs via pkg-config or direct probing, and then generating the corresponding gd pc file

the current cmake/modules/FindWEBP.cmake only tests direct linkage ... it needs to check via PkgConfig first, and if that works, then we can add the result to PKG_REQUIRES_PRIVATES.